Such a LED light source is known from U.S. Pat. No. 7,081,722 B1. The LED loads are LED arrays comprising series arrangements and possibly parallel arrangements of individual LEDs. The known LED light source comprises a rectifier for rectifying the low frequency AC supply voltage. A series arrangement comprising the N LED loads is connected to output terminals of the rectifier. During operation a periodical DC voltage with an instantaneous value varying between zero Volt and a maximum amplitude is present between the output terminals of the rectifier. The known LED light source is equipped with control means for subsequently making the LED loads conduct a current, one by one and starting with a first LED load that is closest to a first end of the series arrangement, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value increases and for subsequently making the LED loads stop conducting a current, one by one and starting with the Nth LED load, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value decreases. These control means typically comprise N control strings, each comprising a transistor and being coupled between the cathode of one of the LED loads and an output terminal of the rectifier.
When the instantaneous value of the periodical DC voltage is zero Volt, all of the transistors comprised in the control strings are conductive but none of the LED loads carries a current. When the instantaneous value of the periodical DC voltage increases, a voltage is reached at which a first LED load and the first transistor comprised in the first control string start conducting a current. Similarly, when the instantaneous value of the periodical DC voltage has increased further to a high enough value, the second LED load and the transistor in the second control string start conducting. In order to minimize power dissipation it is desirable to make sure that the current through the first control string is reduced and preferably stopped.
In the case of a further increase of the instantaneous value of the periodical DC voltage, the remaining LED loads and the transistors comprised in the control strings connected to the cathodes of these LED loads start subsequently to conduct a current. When the nth control string carries a current, the control means ensure that the currents in the first n−1 control strings are reduced or stopped. When all of the LED loads conduct a current, the Nth transistor conducts a current and the instantaneous value of the periodical DC voltage increases further until the maximum amplitude is reached. After that the instantaneous value of the periodical DC voltage starts decreasing. While the instantaneous value decreases the LED loads stop conducting a current one by one in reversed order (first the Nth LED load stops conducting and the first LED load is the last to stop conducting). When the nth LED load stops conducting, the (n−1)th control string starts conducting a current. The nth transistor remains conductive but no longer carries a current or only carries a strongly reduced current. After the first LED load has stopped conducting, all transistors are conductive but none conducts a current, the instantaneous value of the periodical DC voltage decreases further to zero and then the cycle described here-above is repeated. The known LED light source is very compact and comparatively simple. Furthermore, it can be directly supplied from a low frequency AC supply voltage source such as the European or American mains supply.
Several ways to control the currents in the control strings have been disclosed in the prior art. It is for instance possible to make the nth transistor non-conductive when the voltage across the nth control string is higher than a reference value approximately equal to the forward voltage of the (n+1)th LED load. Another possibility is to make the transistor comprised in the nth control string non-conductive when the instantaneous value of the rectified low frequency AC voltage becomes higher than a reference value approximately equal to the sum of the forward voltages of the first (n+1) LED loads. Both these methods suffer from the drawback that the forward voltages of LED loads that are nominally identical show a certain amount of spread. Consequently overlaps or gaps between the conduction intervals of neighboring control strings occur, causing undesirable current spikes and valleys that also reduce the circuit efficiency.
Still another method is to sense the current through the nth control string and make the first n−1 control strings non-conductive, when this current is higher than a reference level. Said method suffers from the drawback that the sensed current signal may become very weak, when the LED light source is in dimmed operation.
Another possibility, illustrated in FIG. 8 of U.S. Pat. No. 7,081,722 B1, is to place an impedance, preferably a resistor, in series with the transistor in each control string and arrange all the resistors in series between the transistor comprised in the first control string and the second output terminal of the rectifier. Furthermore, the LED light source is equipped with a global current control circuit.In the case that the transistors are implemented for instance as NPN transistors, all the base electrodes of these transistors are maintained at the same global current control voltage generated by the global current control circuit. As a consequence, when the transistors in two neighboring control strings for instance the first and the second are both conductive, their emitter voltages are nearly identical (both approximately equal the global current control voltage minus 0.7 Volts (the base emitter voltage drop of a conducting transistor)). However, because of small differences in the characteristics of the transistors comprised in the first and second control string and the unequality of the conducted currents, the emitter voltages are not quite identical. Since the resistor comprised in the first control string is connected between these emitters, the voltage drop across this resistor is nearly zero, so that the current through the first control string is also nearly zero. In a similar way it can be derived that when the nth control string conducts a current, the currents through the first n−1 control strings are reduced with respect to the current through the nth string. A drawback of this method is that the current can never be completely pinched off, but can only be reduced. The reduction factor depends on the current in the nth string, the value of the resistor and the difference of the base-emitter voltages of the involved NPN transistors. This is illustrated by way of an example.
Due to the exponential relation between collector current and base-emitter voltage of a bipolar transistor, a ratio between collector currents of 100 corresponds to a base-emitter voltage difference of some 120 mV at room temperature. In the case that the base emitter-voltages of the transistors in the nth control string and the (n−1)th control string differ 120 mV and the current through the nth string is 10 mA the current through the (n−1)th string will be 0.1 mA=100 μA. The resistance value of the resistor arranged in series with the transistor in the (n−1)th string then is 120 mV/0.1 mA=1200 Ohm. When the current through the nth string decreases, e.g. as a result of dimming, to e.g. 3 mA, the ratio of currents changes as the resistor value is fixed. The resulting current through the (n−1)th string then may be some 60 μA and the ratio between the currents is reduced from 100 to approximately 50. These numerical examples show that the (n−1)th string is not completely cut off, but that the current is only reduced with respect to the current in the nth string. Furthermore these examples show that the ratio between the currents in neighboring strings depends on the dim level of the LED light source.
The fact that the currents through the control strings cannot be decreased to zero and that these unwanted currents depend on the dim level is a disadvantage of this way of controlling the currents through the LED loads and the control strings.
Another effect of said way of controlling the currents through the LED loads is that when a next LED load and a next control string start conducting a current, the current through all of the conducting LED loads increases since the number of resistors comprised in the control strings that the current flows through is decreased by 1 and the global current control voltage generated by the global current control circuit is assumed constant. For this reason this way of controlling the currents is referred to as “phase current stacking”.